The present invention is concerned with glass fibers having high resistance to basic environments, and especially to glass fibers adapted for reinforcement of cementitious products.
Although certain proposals have been made for use of fibers formed of specially formulated glass for the reinforcement of cement products, in general, asbestos fibers are still the most favored reinforcement for such products; but such use increasingly encounters certain difficulties and objections. Thus, it is becoming increasingly difficult to produce asbestos fibers of good quality and, correlatively, the cost of such asbestos fibers is increasing to an extent which is not economically feasible for the users of the products; and still further, the harmful effects of these products in connection with pollution problems is now so well recognized as no longer to require specific demonstration.
For the above reasons, it is becoming imperative to find an alternative solution which is equally satisfactory from the technical and economic viewpoints and which also is not objectionable in relation to pollution problems. For quite a number of years, many experiments have been conducted in an effort to find a substitute reinforcement meeting all of these required conditions. One of the first such experiments was to employ traditional glasses in the formulation of the fibers, for instance, E glass or rock wools. The use of fibers made of these compositions results in failure since the fibers formed of such compositions are rapidly attacked and destroyed by the highly basic environment or matrix resulting from the hydration of cement.
In an effort to overcome this lack of chemical stability, researchers have looked toward formulation of particular glass compositions yielding fibers characterized by the best mechanical and chemical properties. This is especially true with respect to fibers formed of glasses based upon the ternary system--SiO.sub.2 --Al.sub.2 O.sub.3 --MgO, as is pointed out in German Patents DE No. 1,026,928 and DE No. 1,040,444. Although this approach achieved some progress or improvement with respect to fibers made of traditional glasses, nevertheless, the improvement was not sufficient, because these new types of glasses have not been well suited for this particular use. Such glasses were actually developed originally in an effort to obtain greater elasticity and an increased resistance to weathering and aging.
More recently, certain researchers have turned toward still other glass compositions, including those known as zirconium glasses. Glasses of this kind are derived from the system: SiO.sub.2 --Al.sub.2 O.sub.3 --ZrO.sub.2 --RO, RO representing alkaline-earth oxides (Japanese Patent No. 77 04520); and more particularly from the system: SiO.sub.2 --Al.sub.2 O.sub.3 --ZrO.sub.2 --MgO (see U.S. Pat. No. 3,904,424); or possibly from the system: SiO.sub.2 --R'O--R.sub.2 O--ZrO.sub.2, R'O representing alkaline-earth oxides, zinc and/or manganese oxides, and R.sub.2 O representing alkaline oxides (see U.S. Pat. No. 3,861,926).
Various of these compositions intended for use in fiber form most commonly have a high percentage of ZrO.sub.2, usually greater than 10%, and sometimes even as high as 15% or 16%*. This oxide contributes excellent chemical resistance to the glass even in highly reactive alkaline environments; but nevertheless, it does not guarantee adequate chemical resistance of the fibers when such fibers are incorporated in a cementitious product. Moreover, the proposed percentages of ZrO.sub.2 increases the cost or price of the fibers to such an extent as to seriously impair the economic practicality of employing such fibers. FNT *Herein, wherever percentage figures are given, they are to be understood as referring to percentage by weight.
In certain of the prior proposed compositions containing a percentage of ZrO.sub.2 less than 10% (see for example, the glasses described in Japanese Patent No. 77 04520), it has been found that where the content of ZrO.sub.2 is lowered to a value between 5% and 10%, the chemical resistance is extensively impaired. Similarly, in the glasses proposed in French Pat. No. 1,582,318 for adequate resistance to attack of the cement, it was proposed to coat the fibers with a polymer film. These two proposals, therefore, are accompanied by disadvantages which can not be ignored or disregarded.